HIGH-FREQUENCY DIELECTRIC-RELAXATION IN OXYFLUORIDE PEROVSKITE CERAMICS

A strong dielectric dispersion is reported for oxyfluoride ceramics derived from BaTiO3, with the composition Ba(Ti1-xLix)O3-3xF3x(x=0; x=0.025; x=0.05), within the frequency range 10(6)-10(9) Hz. The relaxation is of dipolar type. Cole-Cole analysis leads to a good agreement between theoretical and experimental curves of permitivities epsilon(r)' and epsilon(r)" vs. frequency. The transition sequence rhomb.-orth.-tetr.-cub. which occurs for compositions corresponding to x=0 and x=0.025 becomes a simple rhomb.-cub. transition for x=0.05. Measurements at various temperatures have shown the relaxation frequency to present a minimum at each transition temperature, the deepest minimum is at the Curie temperature. These results are discussed using the lattice-soft-mode phenomena and the model of correlated chains of off-centred M(n+) cations in octahedra (M = Ti, Li).